You know how a cross stitch pattern usually comes in 10x10 block grids on paper? To make your counting easier? Gridding is just repeating that pattern onto the fabric, either using washable pens or lines, or even just using a magic guide fabric, that comes pre-gridded (it comes out when you was the finished piece).
By making a grid (drawing 10x10 or 20x20 squares) on the fabric it's harder to lose count, and for blocks of color you sometimes don't even need to count. It's specially useful on large projects but speeds up even small projects.
Here are some examples:
With fishing line or gridding line:
With washable pen:
Pre-gridded Magic Guide Fabric:
Laura (aka lsschwartz on HAED BB)
WIPs
Krystal (HAED)
Middle Earth Map (HAED)
Snowman&Friends Stocking (Dimensions)
12 Days of Christmas SAL
Little One (Dimensions)